Contemporary engines used in aircraft produce substantial amounts of heat that must be transferred away from the engine in one way or another. Heat exchangers provide a way to transfer heat away from such engines. For example, heat exchangers can be arranged in a ring about a portion of the engine.
Oil can be used to dissipate heat from engine components, such as engine bearings, electrical generators, and the like. Heat is typically rejected from the oil to air by air-cooled oil coolers, and more particularly, surface air-cooled oil cooler systems to maintain oil temperatures at a desired range from approximately 100° F. to 300° F. In many instances, an environment can be as low as −65° F. When this occurs, a bypass valve can be incorporated to allow a portion of the oil to bypass the air-cooled oil cooler and return to the engine without being cooled by the air-cooled oil cooler, if required.
Known bypass valves for use in air-cooled oil cooler systems of aviation engines typically include pressure-actuated valves that are responsive to a change in pressure, and combined pressure and thermally actuated valves that are responsive to a change in both pressure and temperature. Pressure actuated valves are well known in the art, but are not easily tunable. Pressure and thermally actuated valves are also known in the art and typically comprise a valve driven by dual springs and a thermal actuator. The thermal actuator is driven by volume change of a wax contained therein subject to a phase change from a solid to a liquid in response to an increase in temperature. Pressure and thermally actuated bypass valves, while providing a sufficient means for allowing a portion of the oil to return to the engine without being cooled, are unreliable due to sealing issues and the like. In this type of valve, the wax in liquid phase has a tendency to leak and therefore the reliability of the valve is typically lower than desired. In addition, the inclusion of dual springs results in a component of a weight and a cost that can be of issue.